Vehicle position measuring can be performed by using a GPS (Global Positioning System), such as a car navigation system. That is, vehicle position measuring can be performed by a navigation positioning operation using a navigation positioning signal received from a GPS satellite.
In order to perform highly precise positioning even in the environment where it is difficult to receive a GPS radio wave, such as inside a feature blocking the sky or a tunnel, there is developed a GPS-INS integrated positioning which aims at improving accuracy of self-location by equipping a vehicle with sensors, e.g., a vehicle speed sensor, an inertial measuring unit (IMU), etc., to integrate a result of dead reckoning (dead-reckoning navigation) based on these sensors with a GPS so as to perform position measurement (refer to, for example, Patent Literature 1).
In order to obtain a highly precise positioning result in the GPS-INS integrated positioning, it is necessary to measure a speed and a travel distance of a vehicle, with sufficient accuracy. Odometry is used as a means for measuring speed and travel distance. Odometry measures a speed, a travel distance, etc. of a vehicle by detecting a vehicle speed pulse from a pulse signal generated from the rotary encoder attached to the wheel and in accordance with rotation of the wheel, and by integrating the vehicle speed pulses.
As this type of odometry, there is known the one configured by a vehicle speed detection unit in which a rotary encoder is externally attached to the wheel and by a signal processing unit. In the conventional vehicle speed detection unit, the rotor of the rotary encoder is connected to the axle hub of the wheel, and the stator of the rotary encoder is fixed to one end of the hollow shaft. The other end of the shaft penetrates the hole of a holed holding unit, which is protruded to the vehicle outer surface, to be held movably up and down. An output signal of the rotary encoder is transmitted into the vehicle through a signal cable passing in the shaft, and a vehicle speed pulse is processed by the signal processing unit arranged in the vehicle (refer to, for example, Nonpatent Literature 1).
[Patent Literature 1]
    Japanese Unexamined Patent Publication No. 2006-208392[Nonpatent Literature 1]    Product catalog of APPLANIX Corporation (Canada), “Position & Orientation System Land Vehicles” (http://www.applanix.com/products/index.php#land)